Bismuth occurs in metalliferous veins traversing gneiss or clay-slate, and is usually associated with ores of silver and cobalt.
The most certain representatives of the Archean are the gneiss and schists of the Dove river and the upper Forth, and the hornblende-schists, which are exposed in the river valleys on the margins of the central plateau.
This vast area, shaped like a broad-limbed V or U, with Hudson Bay in the centre, is made up chiefly of monotonous and barren Laurentian gneiss and granite; but scattered through it are important stretches of Keewatin and Huronian rocks intricately folded as synclines in the gneiss, as suggested earlier, the bases of ancient mountain ranges.
Dispersed over all parts of the western Highlands, they are most numerous in the north-west, especially in the Outer Hebrides and in the west of the shires of Ross and Cromarty and Sutherland, where the surface of the Archean gneiss is so thickly sprinkled with them that many tracts consist nearly as much of water as of land.
West of the volcanic region, nearer to Lake Victoria and the Eastern province, ironstone, granite, gneiss and schistose formations predominate, with phonolite in places.
The mines there occur in gneiss and schists, which are probably of Archean age; the lode has in places been worked for a width of over 200 ft.
In the mountain region the soil is mostly a sandy loam composed of disintegrated granitic gneiss and organic matter; on the lower and more gentle slopes as well as in the valleys this is generally deep enough for a luxuriant vegetable growth but on the upper and more precipitous slopes it is thin, or the rocks are entirely bare.
Among the commonest associates of the diamond are quartz, topaz, tourmaline, rutile, zircon, magnetite, garnet, spinel and other minerals which are common accessory constituents of granite, gneiss and the crystalline schists.
Eastward of the Archean gneiss in the west of Sutherland the effect of enormous underground pressure has been to upraise masses of the ancient gneiss and Torridonian sandstone and thrust them westward over the younger rocks.
Large ore-bodies of granular and compact magnetite occur as beds and lenticular masses in Archean gneiss and crystalline schists, in various parts of Norway, Sweden, Finland and the Urals; as also in the states of New York, New Jersey, Pennsylvania and Michigan, as well as in Canada.